A conventional power supply system 100 for a vehicle (for instance, JP 10-129402 A2 and JP 08-310337 A2), as illustrated in FIG. 7, comprises a battery 101, a first power supply circuit 102, a second power supply circuit 103, a firing circuit 104, a satellite sensor communication circuit 105, a microcomputer 106, a crash sensor 107, a nonvolatile memory 108 and a backup capacitor 109.
The first power supply circuit 102 steps up or boosts an input voltage vin based on the supply voltage of the battery 101 to provide a predetermined high output voltage Vout (24V). The output voltage Vout is supplied to the firing circuit 104, satellite sensor communication circuit 105 and second power supply circuit 103. The backup capacitor 109 is charged with the output voltage Vout. The second power supply circuit 103 steps down the output voltage Vout to provide a predetermined low output voltage (5V). The input voltage at which the second power supply circuit 103 can stably operate as a 5V regulator is 5.5V. The output voltage of the second power supply circuit 103 is used as power supply voltage for the microcomputer 106, crash sensor 107 and nonvolatile memory 108.
Recently, the power supply system 100 is provided with occupant protecting functions against rear seat side impact and roll over as well as against front impact and front seat side impact. For this reason, the numbers of installed satellite sensors and airbags tend to increase. The processing speed of the microcomputer 106 is increased. For high performance of the power supply system 100, more than one microcomputer 106 may be installed. In this instance, the current consumption in ECUs (Electronic Control Units) is increased. This will result in the increased size and rating of the first power supply circuit 102.
The load on the first power supply circuit 102, that is, the voltage step-up control load is increased with increase in the difference between the input voltage Vin and the output voltage Vout. That is, the lower the supply voltage from the battery 101 is, the higher the voltage step-up control load is. To stably ensure the desired output voltage Vout according to the magnitude of the difference between the input voltage Vin and the output voltage Vout, the size and rating of the first power supply circuit 102 must be increased.
In consideration of the foregoing, another passive safety device is proposed in, for instance JP 09-290704A2 (U.S. Pat. No. 5,995,891), to reduce the current born by the first power supply circuit 102. This device is shown in FIG. 8.
This power supply system 100 is different from that shown in FIG. 7 in that an input voltage detecting circuit 110, a switching element 111 and reverse current blocking diodes 112, 113 are additionally installed. In the input voltage detecting circuit 110, the input voltage Vin is compared with a predetermined threshold value (5.5V). When the input voltage Vin is 5.5V or higher, the switching element 111 is opened (turned off) by a signal from the input voltage detecting circuit 110. For this reason, the first power supply circuit 102 and the second power supply circuit 103 are disconnected from each other. When the input voltage Vin is lower than 5.5V, the switching element 111 is closed by the signal from the input voltage detecting circuit 110. For this reason, the first power supply circuit 102 and the second power supply circuit 103 are connected to each other.
In this power supply system 100, the current for the downstream side of the second power supply circuit 103 need not be ensured at the first power supply circuit 102 when the input voltage Vin is 5.5V or higher. It need not be ensured when the supply voltage from the battery 101 is 6.5V or higher with the voltage drop (1V) at a reverse current blocking diode 112 taken into account. For this reason, the voltage step-up control load on the first power supply circuit 102 can be reduced. Therefore, increase in the size and rating of the first power supply circuit 102 can be suppressed.
However, when the supply voltage from the battery 101 is lower than 6.5V, it is difficult to ensure the operation of the second power supply circuit 103. It is assumed here that the second power supply circuit 103 is operated with the supply voltage from the battery 101 being up to 6V. As described above, the input voltage at which the second power supply circuit 103 can stably operate as a 5V regulator is 5.5V For this reason, in the power supply system 100, the second power supply circuit 103 cannot be operated, when the supply voltage from the battery 101 is within the range between 6V (inclusive) and 6.5V (exclusive) with voltage drop (1V) at the reverse current blocking diode 113.